A known type of brake pressure control device is disclosed in, for example, Japanese Patent Laid-Open Publication No. Sho. 63-64858 published in 1978 without examination. In this known brake pressure control device, a 4-port/2-position electric changeover valve is located between the wheel brake and the master cylinder. The changeover valve is positionable in a first position and a second position. When the changeover valve is in the first position, the wheel brake is brought into fluid communication with the master cylinder such that the wheel brake is isolated from a power pressure source made up of a pump, a pressure accumulator, and other elements. When the changeover valve is in the second position, the wheel brake is brought into fluid communication with the power pressure source such that the wheel brake is isolated from the master cylinder and the master cylinder is brought into fluid communication with an absorbing device or stroke simulator.
The brake pressure control device is also equipped with a depression force sensor which detects the force applied to the brake pedal when the brake pedal is depressed. When the power pressure source is in a normal condition, if the changeover valve is switched from the first position to the second position, the hydraulic pressure outputted from the power pressure source to adjust the hydraulic pressure of the wheel brake becomes responsive to the depression amount of the brake pedal. In contrast, when the power pressure source experiences a failure, the changeover valve is switched from the second position to the first position to establish a direct application of the hydraulic pressure to the wheel brake from the master cylinder.
With the foregoing structure, although the absorbing device or stroke simulator is made up of a piston, a spring, and other elements, the absorbing device has to be provided with a seal member for preventing leakage of the brake fluid. In the event of a breakage of the seal member causing the absorbing device to fail, the changeover valve causes the master cylinder to connect to the absorbing device and disconnect from the wheel brake. Under the resulting condition, if the hydraulic pressure of the wheel brake is intended to be adjusted by using the brake pedal, the brake pedal is advanced through a distance which corresponds to the axial length of a pressure chamber of the master cylinder despite an increase in the depression force applied to the brake pedal. As a result, it becomes impossible to apply the hydraulic pressure to the wheel brake as rapidly as possible.
In light of the foregoing, a need exists for a brake pressure control device for automotive vehicles that is capable of applying brake pressure to the wheel brake as rapidly as possible upon failure of the stroke simulator.